In the existing commercial preparation process of polyolefins, Ziegler-Natta catalysts such as titanium or vanadium compounds have been widely used. However, although the Ziegler-Natta catalyst has high activity, it is a multi-active-site catalyst, and thus, has a limit in securing desired properties because the molecular weight distribution of the produced polymer is wide and the compositional distribution of comonomers is not uniform.
Thus, recently, metallocene catalysts wherein a transition metal such as titanium, zirconium, hafnium and the like, and a ligand comprising a cyclopentadiene functional group are bonded were developed and are being widely used. The metallocene compound is generally activated with aluminoxane, borane, borate or other activators before use. For example, a metallocene compound having a ligand comprising a cyclopentadienyl group and two sigma chloride ligands uses aluminoxane as an activator. Such a metallocene catalyst is a single-site catalyst having one kind of an active site, and has advantages in that the molecular weight distribution of the produced polymer is narrow, and the molecular weight, stereoregularity, crystallinity, particularly reactivity of comonomers may be largely controlled according to the structures of the catalyst and the ligand. However, since polyolefin polymerized with the metallocene catalyst has low melting point and narrow molecular weight distribution, if applied for certain products, productivity may be remarkably decreased due to the influence of extrusion load and the like, and thus, has difficulty in the practical application. Thus, there have been many attempts to control the molecular weight of polyolefin.
Particularly, in order to overcome the above explained problems of metallocene catalysts, many transition metal compounds coordinated with a ligand compound comprising a hetero atom have been introduced. Specific examples of the transition metal compounds comprising a hetero atom may include azaferrocene compounds having a cyclopentadienyl group comprising a nitrogen atom, a metallocene compound of a structure wherein dialkylamine and a silver functional group are connected as additional chains with a cyclopentadienyl group, or a titanium(IV) metallocene compound having an cyclic alkylamine functional group such as piperidine, and the like.
However, among these, just a few metallocene catalysts are practically applied for a commercial process, and thus, there has been a continuous demand for studies on the metallocene compounds that can be used as a polymerization catalyst capable of realizing higher polymerization performance, and providing medium to low molecular weight polyolefin for processability improvement.